There are at least 4 ways to use FATE aspects:
- Tag opponent for bonus to self - you pay 1 Fate
- get tagged by opponent for penalty to self - You get 1 fate
- get compelled by opponent to force a move or prevent a move or attack- you get 1 fate if you accept, pay 1 if not
- get tagged by opponent to who narrates the outcome - You get 1 fate if you accept.
To illustrate these, I'll borrow a scene from Return of the Jedi.
Lando is fighting one of Jabba's henchmen, with Han and Chewie right nearby.
The henchman has just hit Lando.
The henchman wins a maneuver to cause Lando to be Off-Balance. Lando now has that tag.
The GM, realizing that this is just too good to pass up, tags that to lower Lando's initiative below the henchman. Lando's player accepts the fate point. The GM then decides to have some fun, and shoots the skiff. He calls for a roll on athletics to stay upright.
One of the bystanders, however, has a bright idea, and gets the GM's permission to offer a fate chip... to Lando to fall off (a compelled move). Lando's player accepts.
Han's player also blows the roll, and goes prone.
In fairness, the GM rolls for the henchman, who also fails, and decides he's not interesting anymore, so off he goes, too. Han blows a fate, and offers, "Not just down to shoot at Lando, but he fell into the sarlac... munch! munch!" The GM accepts this.
Han decides to grab Lando, but fails the roll. The GM ascribes it to Lando being grabbed by the Sarlac. (Fate point to Lando for narrating a truth about him.)
Han decides to shoot the tentacle, a maneuver to get a free tag on the sarlac, with which to compel a release, but still has that blinded injury. (Injuries can be tagged for free...) Lando helps (by describing Han's aim), and Chewie helps by preventing Han from falling. The GM offers Han's Player a fate to compell him to shoot Lando instead. Han's player thinks for a moment, and says no... So the GM uses the free tag on injuries, and han, of course, misses.
Han tries again the next round, and spends a fate activating his Blaster Master aspect to counter that injury penalty and tries again, and makes it. The Sarlac is labeled, gets compelled by lando, and it lets go. Han makes an athletics to pull him up.
As you can see, it's all about how you offer the FATE. If you offer a compel, it's a gamble... but it's good story. If you tag for a bonus, it's not - pay them, take the bonus, and go.
The one caveat, as I was reminded by Seven Sided Die, is the Authority, be it GM or Table. If a compel or tag is nonsensical, inappropriate, or simply bogs the story, it can be rejected by the Authority. In some FATE games, this authority is the GM's; in others, the authority is the group, and explicitly not the GM's alone. In either case, if an aspect is tagged for bonus, or for penalty, barring "That makes no sense" by the authority, it's a done deal, the fate moves. Compels for specific actions or against specific actions are always subject to review by the authority, and thus have 3 outcomes: Rejected by authority (no fate moves), accepted (Fate to compelled player), or rejected by recipient (Compelled player pays proposer of compel).
Noting that the compel portion can be an absolute bar - for example, blind. Many things, blind is a penalty to. But, for example, to read a book, just compel it with "can't feel the letters, can't see the letters, can't read it."
It's also worth noting that a player rejecting a fate chip for an obviously legit compel probably should be whacked with derision and peer pressure by the rest of the group as it's a clear break from the fiction.